Sex and Evolution (Chapter 11) You’ve seen, in the last lecture, how sexual selection can lead to sexual dimorphism. The example that begins the text chapter identifies an example where genetics plays an important role in the dimorphism, as well as being a major factor in the sex ratio and mating system of a fly species, Cyrtodiopis whitei. This species has stalked eyes; males have varying stalk lengths. The species also has a biased sex ratio. Only around 1/3 of a population are males. With fewer males to ‘service’ the females, any female who could produce more male children would gain fitness. How does a female select a male who will father more sons?

It turns out that male eye stalk length is an indicator. Males with shorter eye stalks produce mostly X bearing sperm, and father daughters, due to defective sperm production. Males with longer eye stalks produce normal sperm ratios, i.e. 50% Y bearing and 50% X bearing. They, therefore, father more sons. In sexual selection within this species, females choose males with longer eye stalks. All this (sexual dimorphism, sexual selection, etc.) assumes that reproduction is occurring sexually.

However, asexual reproduction occurs widely in nature. There are many ways to achieve asexual reproduction, and they have differing genetic consequences. 1)It may occur by budding (as in corals) or by vegetative reproduction (e.g. in plants, by runners (strawberries) or horizontal rhizomes (in many goldenrods). Here every offspring is genetically identical to its parent. The offspring form a clone. 2) It may occur with partial meiosis. The first meiotic division, with crossing over and recombination, produces genetic variation. If there is no second division, the resulting cells are diploid and can develop into mature adults.

3) There can be full meiosis, with fusion of a pair of gametes to restore the diploid number with genetic variation from both meiosis and random gametes fusing. This is one form of self-fertilization. This is asexuality in which only sex (the mating of different individuals) is lacking. Now let’s go back and think about sex. Fitness, in the evolutionary sense, is measured by the number of copies of an individual’s genes, relative to others parents, in the offspring generation. What happens in sexual reproduction? Meiosis and fertilization mean that each offspring carries ½ the genes of each parent.

In asexual reproduction (whichever form) each offspring carries the complete genome of its parent. That is twice as much of a contribution to parental fitness. That difference in genetic contribution between sexual and asexual reproduction is called the cost of meiosis. There have been numerous attempts to study species that can reproduce both asexually and sexually. The object has been to show that females reproducing sexually have twice as many surviving offspring (or more), and (whatever advantages sex offers) they can make up for the cost of meiosis. It has been a failure. Sexual reproduction produces more offspring, but not twice as many. So, why does sex exist and persist?

There is a single answer: variation among offspring. This figure looks across generations (over time). Recombination and crossing over generate variation among offspring of a single set of parents. Both within and across generation variation are important.

If the environment did not differ in time or space, then the advantage of asexuality would predominate, and sex would be rare among species. The asexual parent was successful (grew, survived, reproduced). The adaptations that that the parent had would be equally advantageous for the offspring if the environment remained the same. However, the environment varies over both time and space. Can a parent predict the environmental conditions its offspring will encounter? So, through sexual reproduction (recombination, crossing over) a variety of genotypes (and thus phenotypes) of offspring are produced. At least some should achieve success.

There are two theoretical explanatory constructs: 1. An adaptation of the myth of Sisyphus. Sisyphus was doomed to roll a large boulder uphill. He could never reach the top. The boulder would roll back to the bottom, and he’d have to try again. In evolutionary terms, since the environment is constantly changing, selection can never achieve a ‘perfect’ genotype, and only through sex can evolution ‘keep trying’. 2. The Red Queen hypothesis. Quoting Lewis Carroll: “Here, you see, it takes all the running you can do, to keep in the same place.” Species, to succeed, need to evolve relatively rapidly to succeed in the biological realm, since those they interact with are evolving new defenses, attack strategies, or whatever is important to their success.

If sex is the means to reproduce, what’s the best way to do it? Think about all the problems of finding mates and achieving a successful mating. Wouldn’t it be easier and more successful if male and female functions could be combined in a single individual? A number of species do things that way. They are hermaphroditic. Some are simultaneously male and female (typical in snails and worms and many plants), and some are sequential – first one sex, then the other – (plants and some fish). Sometimes it is years before sex change occurs. In many maples, they are male first (it’s less energetically demanding to be male), then, when they’ve grown larger and stronger they switch to being female. Under harsh, energetically demanding conditions, they may switch back.

However, having both sexes can limit the energy committed to and success of one sex or the other. Evolution will inevitably select the sex strategy that maximizes the sum of success from being male and being female. When having both sexes produces a greater total success than being only one sex, hermaphroditism is advantageous.

If, on the other hand, if each sex interferes with the success of the other one, the sum of successes will be less than could be achieved by having individuals function as only one sex. Hermaphroditism is selected against.

We can only surmise what factors in life history or environment lead to different strategies, but there are many examples of dioecious plants (separate sexes), suggesting that there is some interference in fitness contributions between the sexes… Marijuana is dioecious, and female plants are more valuable; grown in the absence of males, flowers of the plant are called sinsemilla (meaning without seeds).

Here’s another example, with pictures of both female and male plants. It’s a cycad, a relatively primitive gymnosperm. Female Male

Many plants have both sexes separately on the same individual. They are monoecious; the sexes are separate (in different flowers) on the same plant body. Here’s one example, an oak: In some cases male and female flowers appear simultaneously. In others (like the arctic dwarf birch I study), flowers appear sequentially. Sequencing flowers tend to prevent inbreeding.

Finally, some plants produce ‘perfect’ flowers, in which both male and female parts occur together. Even in these flowers the male and female parts may function simultaneously or sequentially. To ensure outcrossing, many plants carry self-infertility genes that prevent inbreeding. These loci must be heterozygous (the alleles at these loci coming from different individuals) for embryos to survive.

Sex Ratios When the sexes are separate, there is a ratio of the number of males to the number of females. We typically think of that ratio as at least approximately 1:1, but that isn’t always the case. When the ratio isn’t 1:1, there is a selective advantage to the minority sex. As a result, the ratio tends to be driven back towards 1:1. This result is called frequency dependent selection.

As a result of differences in the survivorships of the two sexes, the sex ratio may also change with age in a population. Think of the human population. There is a primary sex ratio. It is the number of male and female embryos conceived in a population. The secondary sex ratio is the ratio of the number of male and female babies born. We can assume that the primary sex ratio in humans is 1:1. But we’d be wrong. The primary sex ratio is somewhere between (1948 Carnagie Institute data) and :1. Why? One favorite explanation: Y-bearing sperm are lighter (Y is smaller than X) and more motile. The secondary sex ratio is 1.06:1; 106 male babies for every 100 female babies in the U.S. and Canada. Even so, there is higher in-utero mortality in males than females.

As you’ll see in the demography lectures, mortality through maturation (and even later in life) is also higher in males than in females. The tertiary sex ratio, the ratio at the time of reproduction, is generally slightly female-biased. By middle age and beyond the ratio is even more female biased – among older adults there are more unmated females than males.